1. Field of the Invention
The present invention relates to a driving circuit for a semiconductor device used in a power integrated circuit.
2. Summary of the Prior Art
A bridge inverter for driving a load, such as a motor, by connecting in series two identical voltage-switching elements is known. Such switching elements are connected across a main d.c. voltage source, with that one of the switching elements connected to the high potential side of that source being referred to as an upper arm switching element and the other switching element thereof connected to the low potential side of the voltage source being referred to as a lower arm switching element. By switching those switching elements successively, a drive voltage becomes generated at their point of interconnection and is applied to the load.
To drive the upper arm switching element, it is necessary to apply a driving voltage between a control terminal, which controls the switching of the upper arm switching elements, and the point of connection between the two switching elements. When the lower arm switching element is switched on, the potential of the point of interconnection between the switching elements drops to a level similar to the lower potential side of the main d.c. voltage source. In a similar way, when the upper arm switching element is turned on, the potential of the connection point (or common connection node) of the two switching elements is similar to that of the high potential side of the main d.c. voltage source.
Hence, in order to drive the upper arm switching element, it is necessary to have a separate d.c. voltage source having a standard mid-point potential between the upper and the lower arms.
Consider now the case where it is desired to generate a polyphase signal to the load. In that case, each phase requires a corresponding pair of upper and lower arm switching elements, and the switching elements of the lower arms can be driven from a single power source. However, a separate D.C. insulated power source is required to drive each upper arm switching element. The above driving circuit is described in PCIM'88 Proceedings PP. 32-40 "A high performance Monolithic DMOS bridge for motor drive".
The above mentioned inverter requires a level shift circuit which transfers a signal to a potential equal to the voltage of a main power source applied between the input terminal of a positive power element and the output terminal of a negative power element.
The well-known level shift circuit is not suitable for use in a high voltage difference condition, such as above one hundred volts. Further, there is a trade-off relation between high voltage characteristics or signal transfer speed and the power loss.